SDraw (version 2.1.13)

grts.line: Draw a equi-probable GRTS sample from a linear (line) resource.

Description

Draws an equi-probable unstratified Generalized Random Tessellation Stratified (GRTS) sample from a SpatialLines* object

Usage

grts.line(x, n, over.n = 0)

Arguments

x

A SpatialLines or SpatialLinesDataFrame object.

n

Sample size. The number of sample points to draw from x

over.n

Over-sample size. The number of 'over-sample' points to draw from x. The actual number of points drawn from x is n + over.n.

Value

A SpatialPointsDataFrame containing locations in the GRTS sample, in order they are to be visited. Attributes of the sample points (in the embedded data frame) are as follows:

• sampleID: A unique identifier for points in the sample. This encodes the GRTS ordering of the sample. The output object comes pre-sorted in GRTS order. If the sample becomes un-GRTS-ordered, resort by sampleID (i.e., samp <- samp[order(samp$sampleID),]).

• pointType: A string identifying regular sample points (pointType=="Sample") and over-sample points (pointType=="OverSample").

• geometryID: The ID of the line in x onto which sample points fall. The ID's of lines in x are row.names(geometry(x)).

• Any attributes of the original lines (in x) onto which sample points fall.

Details

This is a wrapper for the grts function in package spsurvey. This simplifies calling grts when equi-probable samples are desired. It extends the allowable input frame types to SpatialLines objects (i.e., no attributes), rather than just SpatialLinesDataFrame objects. For more complicated designs (e.g., variable probability, stratification), call grts directly.

References

Stevens, D. L. and A. R. Olsen (1999). Spatially restricted surveys over time for aquatic resources. Journal of Agricultural, Biological, and Environmental Statistics 4 (4), 415-428.

Stevens, D. L. and A. R. Olsen (2004). Spatially balanced sampling of natural resources. Journal of the American Statistical Association 99, 262-278.

See Also

grts.line, grts.polygon , spsample

Examples

# NOT RUN {
# }
# NOT RUN {
# Draw sample
## The following take > 5s 
HI.sample <- grts.line(HI.coast,10,5)

# Plot
plot( HI.coast )

# Plot 'sample' locations
plot( HI.sample[ HI.sample$pointType == "Sample", ], pch=16, add=TRUE, col="red" )  

# Plot 'over sample' locations
plot( HI.sample[ HI.sample$pointType == "OverSample", ], pch=1, add=TRUE, col="blue" )  
# }
# NOT RUN {

# }